The Siemens 3NJ6940-3BK22 is a pluggable bushing current transformer rated 500 A primary / 5 A secondary with Class 0.5 accuracy. Part of the SENTRON 3NJ6940 accessory family for 3NJ6 in-line fuse-switch-disconnectors, it sits at the precision sweet spot in the 500 A group — tighter accuracy than the Class 1 3BK21, without the calibration overhead of the legal-for-trade 3BK23. The 32 mm cable window and plug-in interface make it a direct fit on any 3NJ6 in-line disconnector.

Why choose Class 0.5 for a 500 A feeder?

A 500 A feeder in a commercial or light industrial building often supplies an entire floor, a production line, or a major HVAC system — loads where knowing the exact consumption drives operational decisions. Class 0.5 accuracy (±0.5% at rated current) supports meaningful internal energy cost allocation: you can confidently attribute kWh to departments, tenants, or processes. If your 500 A circuit simply needs overcurrent confirmation, the Class 1 3BK21 does the job at lower cost. If the kWh reading will appear on an invoice, the calibrated 3BK23 is the only compliant choice.

How do I decide between 500 A, 400 A, and 600 A primary ratings?

Match the CT primary rating to your feeder protection. A circuit protected by a 500 A fuse-switch-disconnector is best served by a 500 A CT — it operates in its specified accuracy band for the majority of the load range. Undersizing (400 A CT on a 500 A feeder) risks saturation and measurement clipping during normal operation. Oversizing (600 A CT on a 500 A feeder) pushes typical operating currents into the low end of the accuracy range, where errors are larger. The 3BK22 is built for feeders fused at 500 A.

500/5 A Accuracy Variant Comparison

Frequently Asked Questions

Is the 3BK22 compatible with third-party energy meters?

Yes. The 5 A secondary and 5 VA burden rating are industry-standard values compatible with virtually all digital and analogue energy meters that accept a 5 A current input. The CT is passive — it does not need to communicate with the meter, it simply provides a scaled current proportional to the primary load. Verify that your meter input impedance plus wiring resistance stays within the 5 VA burden budget at rated secondary current.

Does this CT require any maintenance?

Passive current transformers have no moving parts and require no routine maintenance. The pluggable design means the CT can be removed and replaced without disconnecting the primary cable — useful during disconnector maintenance or replacement. Periodically inspect the secondary terminal connections for tightness, as a loose connection can introduce measurement errors.